Gas turbines are employed in many sectors for the drive of generators or of working machines. In this context, the energy content of a fuel is utilized in order to generate a rotational movement of the turbine shaft. For this purpose, the fuel is burnt in a combustion chamber, compressed air being supplied by an air compressor. The working medium, which is generated in the combustion chamber as a result of the combustion of the fuel and which is under high pressure and under a high temperature, is in this case routed via the turbine unit which follows the combustion chamber and where the working medium expands so as to perform work.
In this case, in order to generate the rotational movement of the turbine shaft, the latter has arranged on it a number of moving blades which are conventionally combined in blade groups or blade rows and which drive the turbine shaft via pulse transmission from the flow medium. Moreover, in order to route the flow medium in the turbine unit, guide vane rows connected to the turbine casing are usually arranged between adjacent moving blade rows. The turbine blades, in particular the guide vanes, in this case usually have, for the suitable routing of the working medium, a blade leaf which is extended along a blade axis and onto which a platform extending transversally with respect to the blade axis can be integrally formed on the end face for fastening the turbine blade to the respective carrier body. However, a platform or a platform-like configuration may also be attached to the other free end.
The design of gas turbines of this type is usually aimed at particularly high efficiency in addition to achievable power. In this case, for thermodynamic reasons, an increase in efficiency can be achieved, in principle, by an increase in the outlet temperature at which the working medium flows out of the combustion chamber and into the turbine unit. Temperatures of about 1200° C. to 1300° C. for turbines of this type are therefore sought after and even achieved.
At such high temperatures of the working medium, however, the components and structural parts exposed to this are exposed to high thermal loads. In order, nevertheless, to ensure a comparatively long useful life of the relevant components, along with high reliability, a cooling of the relevant components, in particular of moving blades and/or guide vanes of the turbine unit, is conventionally provided. The turbine blades are in this case conventionally designed to be coolable, in which case, in particular, an effective and reliable cooling of the leading edge of the respective turbine blade, said leading edge being subjected to particularly high thermal load, is to be ensured.
The coolant used is in this case usually cooling air. This is normally supplied to the respective turbine blade in the manner of open cooling via a number of coolant ducts integrated into the blade leaf or the blade profile. The cooling air, emanating from these coolant ducts, flows, in outlet ducts branching off from the latter, to the regions of the turbine blade which are in each case provided, with the result that a convective cooling of the blade interior and of the blade wall is achieved. These ducts are left open on the outlet side, so that the cooling air, after flowing through the turbine blade, emerges from the outlet ports, also designated as film cooling holes, and forms a cooling air film on the surface of the blade leaf. This cooling air film largely protects the material on the surface against direct and over intensive contact with the hot working medium flowing past at high velocity.
In order to make it possible to have particularly uniform and effective film cooling in the leading edge region of the blade leaf, the outlet ports are conventionally arranged there uniformly along at least two rows oriented parallel to the leading edge. Moreover, as a rule, the outlet ducts are oriented obliquely with respect to the longitudinal direction of the turbine blade, thus assisting the formation of the protective cooling air film flowing along the surface. Since, in the production of the turbine blade, the outlet ducts are normally introduced from outside at the conclusion for cost reasons, for example by laser drilling or other drilling methods, and particularly in the leading edge region of the blade leaf, access for the drilling instrument through the platform or platform-like configurations integrally formed on the end face is possibly obstructed, there is often, with regard to the oblique setting of the outlet ducts, a change in orientation at a transitional point lying approximately centrally between the root section and tip section of the respective blade leaf This takes place in that the coolant flowing out in a root-side subsection of each row possesses, in the region of the outlet ports, a velocity component which points toward the tip section, whereas cooling medium flowing out in a tip-side subsection, contiguous thereto, of each row has a velocity component pointing toward a root section. In other words: in the root-side subsection, the outlet ducts are inclined in the direction of extent to the turbine blade, whereas, in the tip-side subsection, they are inclined opposite to the direction of extent.
Such an arrangement of the outlet ducts may, however, also entail disadvantages. If the change in their orientation and the associated change in the branch-off angle with respect to the coolant duct running in the longitudinal direction and corresponding to the leading edge takes place in a locally abrupt way, then, at the transitional point, possibly relatively large regions between the leading edge and the coolant duct are not penetrated by outlet ducts and therefore also not cooled convectively. This shortcoming then has to be compensated, where appropriate, by cooling air being used to an increased extent in a controlled way. If, instead, the change in orientation of the outlet ducts occurs comparatively continuously, the formation of a film of cooling air flowing along the surface of the blade leaf is impeded in the transitional region, since, there, the cooling air emerges from the film cooling holes almost perpendicularly to the surface and therefore tends to break away from the latter. In this case, too, cooling air has to be supplied to an increased extent, which, in turn, means losses in the available compressor mass flow and diminishes the efficiency of the gas turbine.